(1) Field of the Invention
This invention relates to the coating of metal strips with uniform coating weights and more particularly to the determination or control of the coating weight on hot-metal coated strips with so-called gas doctors or gas knives, and particularly to the use of adjustable orifice gas knives.
(2) Description of the Prior Art
Various means have been used to control the coating weight of so-called hot dip coated strip and sheet material and particularly ferrous-base sheet and strip material which is passed through molten baths of tern, zinc, aluminum, aluminum-zinc and the like. In the coating of such material, the ferrous substrate material as it exits from the bath surface, draws up with it a quantity of at least temporarily adhering molten metal, which, as the ferrous material passes upwardly, solidifies on the surface of the base material. If the strip or sheet material is withdrawn from the bath sufficiently slowly, the excess molten metal will drain from the surface naturally establishing a coating thickness or weight dependent basically upon the temperature of the substrate as well as the coating plus the physical characteristics of the unsolidified coating material, particularly the weight or specific gravity. As coating lines have been accelerated, however, too much excess coating has been drawn up from the molten bath upon the surface of the substrate and has not had sufficient time to drain back into the molten bath causing overly thick as well as uneven coatings on the coated product.
Various expedients have been used to wipe this excess coating from the substrate;surface including charcoal or the like floating upon the bath surface, stationary wipers between which the strip passes, so-called coating rolls between which the strip passes and in more modern practice blasts of gas which wipe the molten coating back into the bath leaving the desired residual surface layer which then solidifies, forming the ultimate coating upon the substrate.
The use of coating or doctor rolls between which the ferrous substrate passed as it exited from the molten bath was a very significant improvement in hot metal coating of sheet and strip material and over a period, the use of such rolls became very sophisticated in design and implementation. These coating rolls essentially limited the amount of metal drawn out of the metal bath with the substrate and then allowed the metal to be recoated with hot metal from the limited reservoir of metal between the rolls. The coating weight could be controlled by the pressure applied between the rolls and the depth the rolls were submerged in the molten bath. Almost all such sophisticated coating roll systems have now, however, been superseded by even more sophisticated gas or fluid wiping apparatus which is more flexible and longer lasting and, in general, does a better job at a lesser cost.
In the fluid or gas Wiping process as used in the steel industry, a gas such as steam, air, nitrogen or other gaseous fluid is forced from a plenum chamber through a narrow slot or slit and directed against a molten metal coated steel strip or sheet as it passes upwardly from the molten coating bath past or by the slot. The narrow elongated blast of gas issuing from the slot wipes excess molten coating from the surface of the metal base and generally smooths the molten surface. Such gas wiping devices are generally referred to as fluid knives or wipers, or, if air is the wiping fluid, usually as air knives or wipers. They are also sometimes referred to as air doctors.
Fluid-wiping techniques have, in turn, developed into very sophisticated systems and a number of apparatus designs have been developed to wipe or smooth various molten coatings including zinc and aluminum and combinations of zinc and aluminum. One persistent problem has been that of so-called edge buildup in which the coating is thicker along the edges of the coated material due generally to less effective wiping at the edges as the result of interference between oppositely directed blasts of gas wiping the two opposite sides of the strip or sheet material. This has necessitated the use of increased slot diameters at the ends of the gas wiping orifices or slots near the sides of the strip or sheet material. However, since the ultimate coating weight is a function of a number of factors, including, primarily, strip or line speed, strip width and surface finish, strip temperature as well as the molten bath temperature, distance of the gas nozzle from the strip, gas flow through the nozzle and configuration of the nozzle, it is difficult to provide and, in fact, so far as it is known, impossible to provide a single nozzle or slot width and configuration that will be suitable for all coating conditions. Superimposed upon these enumerated factors in any particular coating bath is the further complication that each coating material has its own characteristics which inherently change the characteristics of the wiping operation. For example, molten zinc is inherently heavier than molten aluminum so that the effects of gravity tend to be more pronounced and a lesser blast of wiping gas will, in most cases, be effective to remove more molten zinc coating material than aluminum coating material. The amount of edge buildup, furthermore, is likely to be significantly less. Thus, in changing from one to another coating material, a new coating nozzle or wiping air die is usually necessary, requiring not only the changing of nozzles or wiping dies, a not inconsiderable job, but the maintenance of an inventory of different nozzles and dies which inventory is by no means, inexpensive.
Changes in line speed during operation of a coating bath, cooling of the strip, particularly if it has been heated in batch-type rather than in-line furnaces, changes in bath temperature and the like may also have considerable influence upon the wiping of the molten coated surface with a fluid wiper. For example, the molten bath and the strip or sheet being coated are seldom, if ever, of similar temperatures when they are brought together by the strip being immersed in the bath. Consequently, if the strip is cooler, the bath will tend to be cooled and, if the strip is hotter, the bath will tend to be heated as the strip is passed through the bath and the bath will tend to recover in the intervals between strips. This factor in itself causes the coating on the strip to vary progressively in a pattern from end to end. While small, such variation may assume importance for certain products and can be of relatively great economic significance in large scale operations, where any excess coating can assume considerable importance in cost of coating.
It has been recognized, therefore, that it would be desirable to have an infinitely adjustable slot width within normal operating limits which could, in particular, be adjusted while a coating line is operating to make adjustments for other variable factors of the coating operation as well as to make deliberate changes when such appear desirable. For example, it may at times be convenient to change the coating weight part way through the coating of a coil to accommodate a particular order and the like.
It would, in particular, be convenient if the width and configuration of the slot of a gas wiping die could be altered or varied easily, not only between orders or dependent upon coating conditions, but also as a coating operation progresses. Among prior attempts to provide variable orifices to conveniently make changes in the coating of a substrate may be mentioned the following:
U.S. Pat. No. 2,415,644 issued Feb. 11, 1947 to L. W. Leonhard et al. discloses an early form of doctor blade or air knife for smoothing and wiping the coating on a strip or web. Changes in the coating weight obtained by changing various parameters of the coating process, including the gas pressure, the width of the gas wiping orifice and other expedients are mentioned or discussed. Leonhard et al. discloses an air knife having two halves held together by adjusting and locking bolts. These bolts extending through the center of the enclosed chamber may in one embodiment be adjusted by a mechanical arrangement to adjust the opening of the gas orifice between the two halves of the die body in accordance with a control system. While the Leonhard et al. apparatus is designed especially for coating nonmetallic webs and the like and is arranged to operate on only one side of the web, it would probably be applicable also to the coating of metal strip and sheet material.
U.S. Pat. No. 3,753,418 issued Aug. 21, 1973 to R. Roncan discloses a wiping die arrangement for ferrous sheet and strip incorporating a nonadjustable upper lip opposed to an adjustable or flexible lip. The flexible lower lip may be adjusted by means of a mechanical screw arrangement which, through a series of rotating arms and intermediate inter-meshing gears, forces the flexible lip up and down to adjust the distance between the lips. Differential adjustment along the length of the lip is said to be attainable.
U.S. Pat. No. 3,841,557 issued Oct. 15, 1974 to E. S. Atkinson discloses a gas wiping die in which the width of the slot from which the gas issues can be selectively adjusted by means of heating elements positioned at closely spaced intervals in one-half of the die. Atkinson, in his background discussion, discloses prior use of removable shims or inserts and the use of adjusting screws or bolts to warp the walls of the gas slot with respect to each other. These prior methods were slow and required an excessive amount of labor, however.
U.S. Pat. No. 3,917,888 issued Nov. 4, 1975 to D. J. Beam et al. discloses a gas wiping die in which the orifice width can be selectively changed by sliding into place on one lip, a variable insert member. The rate of taper of the opening can be adjusted by inserting different adjustment strips on the lip.
U.S. Pat. No. 3,938,468 issued Feb. 17, 1976 to J. B. Kirschner discloses a gas wiping die having a variable slot valve means mounted in or between the lips of the gas orifice of the die to change the effective cross section of the orifice. The valve means is in the form of a long, substantially round member or bar mounted in a circular opening within the die lips. The circular member is relieved in a certain pattern along one side to form a variable valve surface to allow the escape of gas past the rotatable valve member.
U.S. Pat. No. 4,106,429 issued Aug. 15, 1978 to I. J. Phillips discloses a gas wiping die in which the wiping orifice may be varied in size by moving the lips of the wiping orifice of the die longitudinally toward or away from the material being wiped. The lips are moved longitudinally with respect to the remainder of the wiping die. Such longitudinal movement is effected by a rotatable cam arrangement journaled within the lip structure itself. When a cam is rotated, it causes the separate lip structure to slide laterally with respect to the remaining structure of the wiping die, in effect, lengthening or shortening the lips.
U.S. Pat. No. 4,524,716 issued Jun. 25, 1985 to H. J. Mueller discloses a recent gas wiping die particularly for use in the paper coating arts. The Mueller die has converging lips and is provided with a flexible gas flow modification member in the form of a semi-stopper directly behind the opening of the lips. This stopper may be moved toward and away from the opening in the lips to relatively open or close the orifice and thereby control the flow of gas from the lips.
As can be seen from the above citations of prior patented gas wiping dies, there have been a fair number of attempts to produce a practical wiping die having a variable, and preferably, an infinitely variable die orifice. However, each has had certain disadvantages and no really satisfactory arrangement for varying the contour of the lips of the gas die to vary the flow of gas through various portions of the gas die to allow for varying conditions on a coating line have been either designed or developed. There is, consequently, a need for a practical gas wiping die or gas doctor, particularly for wiping and smoothing molten metal on the surface of ferrous strip and sheet material that may be conveniently varied
in cross section to quickly vary the amount of gas issuing from the die at any given location across the face of the die to allow for variations in the operation of the coating line.